Fix MA0050 code fixer producing an extension local function (CS1106)

[meziantou]

Fixes #1325

What changed

ValidateArgumentsCorrectlyFixer.CopyParametersWithoutDefaultValues stripped default values but kept every parameter modifier when building the generated local function's parameter list — including this.

The analyzer accepts extension methods: its only parameter guard is Parameters.All(p => p.RefKind == RefKind.None), and an extension receiver has RefKind.None. So applying the fix to an extension iterator method generated a local function declared as an extension method, which does not compile:

error CS1106: Extension method must be defined in a non-generic static class

The method now also removes the this modifier, and was renamed CreateLocalFunctionParameterList since it no longer only strips defaults.

Why params and scoped are left alone

The issue suggested defensively stripping params and scoped as well. I did not:

• params is legal on a local function, and the forwarding call passes the array through directly, so keeping it is both valid and semantics-preserving.
• Stripping scoped would be a regression rather than a defense — passing a scoped argument to a non-scoped parameter is a ref-safety error. (Moot in practice, since iterator methods cannot have ref struct parameters.)

Only this is actually invalid on a local function, so only this is removed.

Test

Added ReportDiagnostic_ExtensionMethod using the repro from the issue — there was no extension-method fixture in the existing tests.

I verified the test actually guards the regression: with the fixer reverted it fails (the harness compiles the fixer output, so CS1106 surfaces), and it passes with the fix in place.

Verification

• All 17 tests of ValidateArgumentsCorrectlyAnalyzerTests pass on every supported Roslyn version (4.8, 4.14, 5.0, 5.6, 5.9).
• dotnet run --project src/DocumentationGenerator exits 0 with no markdown changes, as expected for a code-fixer-only change.